Continuous inkjet printing is a printing technology that is well suited for high-speed printing applications, having high throughput and low cost per page. Recent advances in continuous inkjet printing technology have included thermally induced drop formation, which is capable of selectively altering the drop breakoff phase relative to a periodic charging electrode waveform and thereby controlling whether the drop is charged or uncharged, and electrostatic deflection of charged drops to separate the charged non-print drops from the uncharged print drops. These advances have enabled the print resolution to be significantly improved while maintaining the throughput of the printer.
As discussed in commonly-assigned European Patent 1013424, drop charging and deflection depend on the charging voltage and the spacing between the charging electrode and the liquid streams from which the drops break off. Deviations in charging electrode flatness across the length of the nozzle array can therefore result in variation in impact height of the non-print drops on the catcher. Such variations in impact height tend to reduce the operating latitude of the printhead. As noted in commonly-assigned U.S. Pat. No. 7,163,281, the heating of the charging device to prevent condensation on the charging device can thermally deform the charging device altering the spacing between the charging electrode and the liquid streams, and thereby affecting the operating latitude of the printhead.
As discussed in commonly-assigned U.S. Pat. No. 7,156,488, when printheads have reduced nozzle sizes, which is desirable for higher quality color printing, the operations for removing contaminants from sensitive components can leave ink in the gap between the charging device and the nozzle plate. Failure to remove ink from this space can result in electrical shorting conditions between any exposed conductive traces on the upper surface of the charging device and other conductive surface in the printhead. These types of shorting conditions often result in printhead errors and premature printhead failure. To prevent such electrical shorting conditions, prior art systems have typically applied an insulating layer such as an insulating epoxy layer over the conductive traces on the upper surface of the charging device. While such insulating layers do provide protection for the conductive traces on the charging device, the presence of the insulating layer on the upper surface of the charging device reduces the size of the gap between the charging device and the nozzle plate which can further impede the removal of ink from the gap between the charging device and the nozzle plate. Furthermore, under prolonged exposure to the ink, the insulating epoxy layers have been found to degrade.
There remains a need for an improved charging device construction that provides very uniform drop charging and deflection across the nozzle array, that undergoes minimal thermal deformation during operation, and that provides superior insulation of the charging electrode conductive traces without encroaching into the gap between the charge device and the nozzle plate.